This invention relates generally to the field of resistive elements used in variable resistors (potentiometers and rheostats). More particularly, it relates to an improved cermet resistive element that provides lower contact resistance and improved contact resistance stability, while maintaining good linearity, resolution, setability, and wear characteristics.
Resistive elements made of thick film cermet inks have achieved widespread usage in the electronics industry. See, for example, U.S. Pat. Nos. 2,950,995; 3,149,002; 3,200,010; 3,207,706; 3,252,831; 3,308,528; 3,326,720; 3,343,985; 3,479,216; and 3,573,229. In fabricating cermet resistive elements, there are several criteria that are sought to be achieved. For example, it is desirable to minimize contact resistance while also maximizing durability (in terms of element and wiper life), thermal stability, resolution, and setability. To a large extent, there must be some trade-off among these goals, especially if cost is a factor.
The contact resistance of a variable resistor is a resistance that is exhibited between the conductive wiper and the resistive element. The contact resistance is actually the sum of two components: a "constriction" resistance and a "tarnish" resistance. The former is proportional to the sum of the wiper and resistive element resistivities, and it is inversely proportional to the effective diameter of the surface-to-surface contact between the wiper and the resistive element. The latter is a function of the resistivity of contaminants or oxide films which may occupy the contact area.
The prior art has taken a number of approaches toward minimizing contact resistance. A common approach is the use of multi-fingered wipers to increase the number of contact points. To reduce the "tarnish" resistivity, noble metals are used in the wipers. Both of these approaches add appreciably to the expense of manufacture. Another approach is to increase the force of the wiper against the resistance element. This technique, however, reduces the life expectancy of the wiper and the resistive element. The prior art has also employed chemical and mechanical (i.e., abrasive) means to remove surface irregularities and contaminants on the resistive film. Such surface treatments, however, may create changes in the total resistance of the element and induce instabilities.
An approach that has shown some promise is that of lowering the surface resistivity of the resistive element at the point of contact with the wiper, without significantly lowering the bulk resistivity of the element, thereby maintaining the desired total resistance. An example of such a technique is disclosed in U.S. Pat. No. 3,717,837 to MacLachlan. The technique disclosed therein comprises the vapor deposition of noble metal (e.g., gold, palladium, rhodium, or platinum) onto the surface of a vapor-deposited cermet resistive element. The result, as stated in the MacLachlan patent, is to decrease contact resistance while only slightly lowering the total resistance of the cermet element.
While the approach taught by the MacLachlan patent shows promise, there are still drawbacks. For example, the random shape, size, and distribution of the conductive metal "islands" on the cermet surface can result in problems of resolution, linearity, and setability, due to an uneven distribution of the islands, and due to the irregular shapes and sizes of the islands. Moreover, the high conductivity of the vapor-deposited noble metal requires careful control of its application in order to avoid unwanted shunting of the resistive element and the resultant excessive lowering of its total resistance.
It can thus be appreciated that the prior art, while recognizing the need to lower contact resistance, has not yet developed the ability to do so without seriously compromising other important design considerations.